Pulsed directed energy deposition based fabrication of hybrid titanium/aluminum material for enhanced corrosion resistance and strength

ABSTRACT

A method of providing a protective titanium layer to an outer surface of an aluminum component includes providing an aluminum component and forming a first layer of titanium-based bulk metallic glass on the component, wherein formation of the bulk metallic glass layer comprises depositing a titanium alloy powder using pulsed directed energy deposition.

BACKGROUND

The present application is directed generally to the fabrication ofhybrid titanium/aluminum components and more particularly to a methodand material for providing enhanced corrosion resistance and strength toan aluminum component.

Titanium alloys have been applied to aluminum components, particularlythe leading edges of a fan blades, to enhance strength and provideprotection from erosion. Current methods of manufacture of titaniumexternal elements and assembly are complicated. Because titanium andaluminum are galvanically incompatible, the two materials generally mustbe separated by an insulating layer to limit corrosion. The fabricationof a titanium sheath on a hollow aluminum fan blade requires amulti-axis machining process. Application of the sheath can causedistortion of the blade and post processing can be required to removedefects. The current processes can be material- and time-intensive.

A need exists for an improved method of providing a titanium protectivelayer to an aluminum component.

SUMMARY

In one aspect, a method of providing a protective titanium layer to anouter surface of an aluminum component includes providing an aluminumcomponent and forming a first layer of titanium-based bulk metallicglass on the component, wherein formation of the bulk metallic glasslayer comprises depositing a titanium alloy powder using pulsed directedenergy deposition.

In another aspect, an aluminum component includes an outer surface and asheath covering at least a portion of the outer surface, wherein thesheath comprises a titanium-based bulk metallic glass layercharacterized by a predominantly amorphous microstructure.

In yet another aspect, a method of providing a protective titanium layerto an aluminum component includes depositing a titanium alloy powder onan outer surface of the aluminum component, pulsing energy directly tothe titanium alloy powder, melting the titanium alloy powder, andforming a first layer of titanium-based bulk metallic glass on thecomponent.

The present summary is provided only by way of example, and notlimitation. Other aspects of the present disclosure will be appreciatedin view of the entirety of the present disclosure, including the entiretext, claims, and accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an assembly for providing a protective titanium layer to analuminum component.

FIG. 2 is a perspective view of an aluminum fan blade of a gas turbineengine having a protective titanium sheath applied to a leading edge.

FIG. 3 is a cross-sectional view of one embodiment of the protectivetitanium sheath of FIG. 2.

FIG. 4 is a flowchart of a method for forming a titanium element on analuminum component.

While the above-identified figures set forth embodiments of the presentinvention, other embodiments are also contemplated, as noted in thediscussion. In all cases, this disclosure presents the invention by wayof representation and not limitation. It should be understood thatnumerous other modifications and embodiments can be devised by thoseskilled in the art, which fall within the scope and spirit of theprinciples of the invention. The figures may not be drawn to scale, andapplications and embodiments of the present invention may includefeatures, steps and/or components not specifically shown in thedrawings.

DETAILED DESCRIPTION

The present disclosure provides an improved method of manufacture of ahybrid titanium/aluminum component. Using directed energy deposition(DED), a titanium alloy can be directly deposited onto an aluminumsubstrate. The disclosed process produces a titanium-based bulk metallicglass (BMG) having an amorphous microstructure, which has been shown toresult in a 3-5 order of magnitude reduction in electrical conductivityover current titanium sheaths. Crystalline phases are introduced locallyinto the BMG by ultrasonic vibration. The introduction of crystallinephases has been shown to increase ductility of the titanium element. Thedisclosed BMG with selectively introduced crystalline phases can providestrength and protection to the component while reducing or eliminatingthe need for an insulating layer.

The disclosed method is not limited to the fabrication of a particularcomponent. In addition to providing a titanium sheath on a portion of ablade (e.g., airfoil leading edge, root, etc.), the disclosed method canbe applied for the fabrication of gears, heat exchanger parting sheetsand headers, and titanium blades on an aluminum hub (e.g., hybridmaterial impellers), among a wide variety of other applications. It willbe understood by one of ordinary skill in the art that the disclosedmethod could benefit multiple aluminum fabrication and assemblyapplications. Direct deposition of the titanium-based BMG can reduce theneed for post-processing of aluminum components, including complexmachining and application of an insulating layer, before assembly with atitanium element.

FIG. 1 illustrates a modified DED assembly 10 with ultrasonic vibrationfor providing a protective titanium layer to an aluminum component. DEDis a metal additive manufacturing process in which metal powder isdelivered to a substrate or workpiece as opposed to being provided in apowder bed. Metal powder is fed through a nozzle while an energy source(e.g., electron beam or laser) melts the powder to provide a melt pool,which is deposited on the component in a layer-by-layer fashion.

Assembly 10 includes a conventional DED system that has been modified toprovide ultrasonic vibration to the substrate during deposition.Assembly 10 includes table 12, ultrasonic transducer 14, depositionplatform 16, energy source 18, powder nozzle 20, power supply 22, andcontroller 24. Assembly 10 can be used to deposit thin layers oftitanium-based BMG on substrate 26. Substrate 26 can be positioned andsecured on deposition platform 16 in a manner that allows for materialdeposition on an outer surface of substrate 26. Table 12 can beconfigured as known in the art to be moveable in x-, y-, and z-planes tomove substrate 26 relative to powder nozzle 20 and energy source 18during deposition. In alternative embodiments, energy source 18 andpowder nozzle 20 can be positioned on a multi-axis arm and can moverelative to deposition platform 16 and substrate 26.

Power supply 22 can be used to drive ultrasonic transducer 14.Controller 24 can be used to automate the deposition of thetitanium-based BMG on substrate 26 according to a user-defineddeposition model. Controller 24 can turn ultrasonic transducer 14 on andoff to provide localized production of crystalline phases in thedeposited material and can also control movement of table 12 and otherprocess parameters. Controller 24 can include a processor configured toimplement and/or process instructions for execution stored in a storagedevice (e.g., computer-readable storage media).

Substrate 26 can be formed of aluminum or an aluminum alloy. Substrate26 can be a net shape or near net shape component formed by casting,forging, additive manufacturing, or other fabrications processes asknown in the art. In one embodiment disclosed herein, substrate 26 is ahollow aluminum fan blade (illustrated in FIG. 2). Substrate 26 is notlimited to the embodiment disclosed. It will be understood by one ofordinary skill in the art that substrate 26 can be any aluminumcomponent to which a titanium element may be applied to provideprotection and strength to the underlying structure.

A titanium-based BMG powder (or titanium alloy capable of forming a BMGupon deposition with DED) can be deposited on substrate 26 using aconventional pulsed laser DED system. The powder size can generally bedictated by the operating capabilities of the machine. In the disclosedembodiment, the powder size ranged from 30 to 70 μm. The titanium-basedBMG powder can be applied in multiple successive layers. Followingformation of each layer, table 12 can be moved downward (z-direction) tomaintain the focus of the laser at the surface. A single powdercomposition or multiple powder compositions can be fed through a singlepowder nozzle or multiple powder nozzles, as known in the art. Layerthickness can vary based on powder size and processing parameters. Inone non-limiting embodiment, layer thickness was approximately 20 μm.

A variety of titanium alloys form a BMG upon pulsed laser deposition.Preferably, a titanium-based BMG powder used in the method disclosed hasa melting temperature that does not exceed a melting temperature of theunderlying substrate by more than 125 K. This allows for control of thedepth of the titanium-based BMG melt pool interface with the outersurface of the aluminum substrate 26 during deposition of the firstlayer of the titanium sheath. In one non-limiting embodiment, thetitanium-based BMG is Ti₄₀Zr₂₀Cu₁₀Be₃₀, which has a melting point of1000 K (75 K higher than pure aluminum). By controlling the table speedand laser power, a very thin layer of interaction between thetitanium-based BMG and aluminum can be formed.

Although rare, interaction between a melted layer of the aluminumsubstrate 26 and the titanium-based BMG may result in the undesirableproduction of an aluminide. To avoid aluminide formation, someembodiments may include the deposition of a layer of silver, magnesium,or other metal that does not interact with aluminum or titanium alloys,directly onto substrate 26 before deposition of the titanium-based BMG.The introduction of silver, magnesium, or other metal may additionallyimprove adhesion between the titanium-based BMG and substrate 26,although excellent adhesion between aluminum substrate 26 and thetitanium-based BMG Ti₄₀Zr₂₀Cu₁₀Be₃₀ was observed with the disclosedprocess parameters. A powder of silver, magnesium, or other metal can beprovided in the DED process as a single layer separate from thedeposition of the titanium-based BMG and/or can be introduced with thetitanium-based BMG in decreasing amounts in successive layers. Thedisclosed interlayer may not be necessary in all applications and,therefore, may be omitted from the disclosed method without negativeimpact. Generally, a high laser power and low travel speed can promoteformation of an aluminide layer. It will be understood by one ofordinary skill in the art to provide the disclosed interlayer where theformation of aluminide is likely or to improve adhesion.

The titanium-based BMG can be applied using a standard pulsed laser asprovided in a conventional DED system. The disclosed method wasdeveloped using a standard laser frequency of 36 kHz. It is anticipatedthat a much lower pulsed laser frequency could be applied and providethe same results. The pulsed laser, as opposed to continuous laser,promotes rapid cooling, which is necessary for the formation of theamorphous BMG.

The grain morphology of a particular material is influenced by the DEDprocessing parameters. To promote amorphous or planar grain structures,the solidification rate must be kept relatively low and temperaturegradient high. The solidification rate is generally reflected in thetable speed in the x- and y-directions. The temperature gradient isinversely proportional to the power of the laser. As such, table motionspeed and laser power can be selected to provide the rapid coolingnecessary for formation of the amorphous structure. The predictedcooling rate required to fabricate an amorphous titanium-based BMG is10⁵-10⁶ K/s.

While the amorphous titanium-based BMG has been demonstrated to begalvanically compatible with an aluminum substrate, BMGs in general canbe more brittle. To improve ductility of the titanium element,crystalline phases can be introduced into the titanium-based BMG.Crystalline phases are induced during the deposition process through theapplication of ultrasonic vibrations to substrate 26. Ultrasonictransducer 20 can be positioned under platform 14 upon which substrate26 is secured. The ultrasonic vibrations provide more seeds which drivecrystalline grain growth. The introduction of ultrasonic vibrations canthereby produce a hybrid microstructure including both crystalline andamorphous microstructures. Multiple crystalline phases can be introducedin a single layer and in multiple layers in varying amounts by turningultrasonic transducer 20 on and off. Crystalline phases increaseconductivity of the material and thereby can make the material moresusceptible to corrosion. As such, it will be understood by one ofordinary skill in the art to limit the location of crystalline phaseswithin the titanium element to regions where corrosion is less likely tooccur.

In some embodiments, process parameters can be selected to promotecrystalline growth. Process parameters can be selected to provide ahigher solidification rate and lower temperature gradient than requiredfor amorphous structure formation. For example, higher laser power andincreased table speed can be used to promote crystalline growth. Processparameters can be modified to promote crystalline growth independent ofor in conjunction with the addition of ultrasonic vibrations.

Controlling grain structure with processing parameters can require anaccurate material model, whereas use of ultrasonic vibration is moreheuristic in nature. An accurate thermal model of the material andexperimental data is needed to calibrate a processing map throughmultiple process parameter variations and to obtain an accurate materialmodel for crystalline-amorphous transition. In contrast, it is giventhat higher ultrasonic frequency than a threshold value (which can bedetermined from experiments) will induce more seeds and initiatecrystalline transition.

In some embodiments, deep rolling, including ultrasonic deep rolling, asknown in the art, can be introduced to further improve mechanicalproperties of the component. Deep rolling induces compressive stress tothe component, which can further improve ductility and fractureproperties. Deep rolling can be applied to each deposited layer of thetitanium-based BMG or can be applied to select layers as needed toobtain the desired material properties for a particular application. Ina non-limiting embodiment, ultrasonic deep rolling at a frequency notexceeding 50 kHz helped promote a desired ductility of at least 10%elongation.

FIG. 2 is a perspective view of a fan blade of a gas turbine enginehaving a protective titanium sheath applied to a leading edge using thedisclosed pulsed laser DED method with ultrasonic vibration. FIG. 2illustrates fan blade 28. Fan blade 28 includes a hollow aluminum fanblade (substrate 26) with outer surface 30 and titanium sheath 32applied at leading edge 34. Titanium sheath 32 is a multi-layertitanium-based BMG formed by pulsed laser DED with crystalline phasesinduced by ultrasonic vibrations in core layers to improve ductility.Titanium sheath 32 can be formed to any desired thickness. In thedisclosed embodiments, the titanium sheath 32 can provide protection andstrength to an underlying aluminum blade at least equivalent to aconventional titanium sheath of the same thickness.

FIG. 3 is a cross-sectional view of one embodiment of the protectivetitanium sheath of FIG. 2. FIG. 3 illustrates substrate 26, optionalinterlayer 36, predominantly amorphous titanium-based BMG layer 38,titanium-based BMG with crystalline phases layer 40, and predominantlyamorphous titanium-based BMG outer layer 42. Each layer 36, 38, 40, and42 can include a single material deposition layer or multiple layers ofmaterial deposition to achieve a desired thickness.

Interlayer 36 can optionally be applied directly to substrate 26 toprovide an interface between the aluminum substrate and titanium-basedBMG. Interlayer 36 can be magnesium, silver, or other metal that doesnot galvanically interact with aluminum and titanium. Interlayer 36 canlimit the formation of undesirable aluminides during the DED process andmay improve adhesion of titanium-based BMG layer 38. Interlayer 36 canbe deposited as a single layer of pure material (e.g., magnesium) or canbe deposited in multiple layers in combination with the titanium-basedBMG layer 38, where the interlayer material is deposited in decreasingamounts in each successive layer. For example, BMG layer 38 can includea plurality of sub-layers (not labelled) in which interlayer materialcan be included in decreasing amounts from substrate 26 outward.Interlayer and titanium-based BMG powders can be deposited throughseparate powder nozzles or can be premixed and deposited through asingle nozzle.

Titanium-based BMG layer 38 can include multiple layers of materialdeposition. Titanium-based BMG layer 38 is applied by pulsed laser DEDwith table speed and laser power selected to provide rapid cooling topromote BMG formation. Titanium-based BMG has a predominantly amorphousgrain structure, which limits galvanic corrosion. In one non-limitingembodiment, titanium-based BMG can be Ti₄₀Zr₂₀Cu₁₀Be₃₀, which has amelting point of 1000 K (75 K higher than pure aluminum). By controllingthe table speed and laser power, a very thin layer of interactionbetween the titanium-based BMG and aluminum can be formed. Othertitanium-based BMGs are known in the art and may be suitable forapplication to aluminum. Selecting a titanium-based BMG having a meltingpoint not exceeding 250 K greater than the aluminum or aluminum alloycan limit the thickness of the layer of interaction formed between thesubstrate and first layer of deposited material.

Titanium-based BMG layer 40 having crystalline phases is formed overtitanium-based BMG layer 38. The crystalline phases are provided toincrease ductility of the titanium sheath. In the non-limitingembodiment illustrated, sufficient crystalline phases are introduced toprovide a ductility of at least 10 percent elongation. Layer 40 caninclude multiple layers of material deposition with similar or varyingamounts and locations of crystalline phases formed in each layer.

Layer 40 can be formed by depositing the titanium-based BMG in a mannerconsistent with the deposition of titanium-based BMG for layer 38, butwith the addition of ultrasonic vibration. Crystalline transition orgrain refinement is induced by ultrasonic vibration provided byultrasonic transducer 20 to substrate 26 during material deposition.Ultrasonic transducer 20 is turned on to induce crystalline phases intothe microstructure and turned off to promote amorphous grain structure.In some embodiments, ultrasonic vibrations can be provided continuously(ultrasonic transducer 20 remains on) during material deposition of alayer. In alternative embodiments, ultrasonic vibrations can be appliedintermittently to promote a hybrid microstructure including bothamorphous and crystalline microstructures. Grain structure control andoptimization can be provided by applying and removing ultrasonicvibration during the deposition process according to a programmed modelof the microstructure.

In some embodiments, layer 40 can be formed by depositing thetitanium-based BMG using process parameters that produce a highersolidification rate and/or lower temperature gradient than needed foramorphous structure formation, thereby, promoting crystalline growth. Insome embodiments, both processing parameter selection and application ofultrasonic vibration can be used to induce crystalline microstructures.

Surface layers are prone to environmental degradation and hot corrosion.As such, it can be desirable to provide a second layer 42 oftitanium-based BMG for the outermost portion of titanium sheath 32thereby sandwiching crystalline layer 40 between two amorphous BMGlayers 38 and 42. Titanium-based BMG layer 42 can be formed in a mannerconsistent with the formation of titanium-based BMG layer 38, asdiscussed above. The presence of crystalline layer 40 can providesufficient ductility to titanium sheath 32.

Titanium-based BMG can provide a 3-5 order of magnitude reduction inelectrical conductivity, thereby reducing the potential for corrosion,while also providing high strength to leading edge 34. The electricalconductivity for bulk metallic glass titanium alloys of interest isaround 5.800×10² Siemens/m as compared to conventional non-bulk metallicglass titanium alloys, which have an electrical conductivity around5.800×10⁵ Siemens/m.

FIG. 4 is a flowchart of method 50 for forming a titanium element on analuminum component. An aluminum component in net shape or near net shapeform is secured to a deposition platform in step 52. The aluminumcomponent can be formed by any method known in the art, including butnot limited to casting, forging, and additive manufacturing. Thealuminum component can be formed of aluminum or an aluminum alloy. Thealuminum component is not limited to a particular component disclosedherein but can include any component for which a titanium sheath orprotective outer layer is applied.

An optional layer of magnesium, silver, or other metal compatible withboth aluminum and titanium can be deposited on the aluminum substrate tolimit the formation of aluminides and to promote adhesion between thealuminum component and a titanium-based BMG layer (step 54). Theinterlayer can be deposited in a single layer or multiple layers usingDED. The interlayer can be formed of pure magnesium, silver, or similarmetal or can be mixed with the titanium-based BMG in decreasing amountsin successive layers.

The titanium-based BMG layer is deposited on an outer surface of thealuminum component or the interlayer using a conventional pulsed laserDED system in which powder is supplied to the surface of a substrate ina focal beam of a laser and melted to form a BMG layer (step 56). Atable moves the aluminum component in x- and y-directions to form auniform layer over an entirety of an outer surface or specified regions.Processing parameters, including laser power and table motion can beselected to provide rapid cooling to promote formation of the amorphousBMG microstructure. With each layer, the table is moved downward tomaintain laser beam focus on the outermost surface of the component.

The titanium-based BMG material can be selected based on mechanicalproperties and melting point. Deposition of a titanium-based BMG with amelting point not exceeding 125 K of the melting point of the aluminumsubstrate can limit a thickness of the region of interaction with thealuminum substrate during the deposition process. The titanium-based BMGlayer can include multiple layers of deposited material.

Crystalline phases can be introduced in a subsequent layer oftitanium-based BMG to improve ductility of the protective titaniumelement (step 58). The formation of crystalline phases is induced by theapplication of ultrasonic vibrations to the aluminum component duringmaterial deposition. Ultrasonic vibrations can be provided by anultrasonic transducer positioned below the deposition platform. Acontroller can be used to automatically turn the ultrasonic transduceron and off according to a model of the component microstructure. Grainstructure of the protective titanium element can be optimized throughmodeling and can be controlled by applying and removing ultrasonicvibration during the deposition process and selecting processingparameters. Ultrasonic vibrations can be applied continuously to promotecrystalline grain structure throughout a layer or can be appliedintermittently to promote a hybrid microstructure having bothcrystalline and amorphous microstructure.

An additional amorphous titanium-based BMG layer is deposited on theoutermost portion of the titanium sheath to limit degradation and hotcorrosion (step 60). The outer titanium-based BMG layer can be formed ina manner consistent with the formation of inner titanium-based BMGlayer, as discussed above.

Deep rolling can be applied following deposition of any material layer,including the final amorphous titanium-based BMG layer formed in step60, to improve the mechanical properties of the protective titaniumsheath (step 62). Deep rolling induces compressive stress to thecomponent, which can further improve ductility and fracture properties.Deep rolling can be applied to each deposited layer of thetitanium-based BMG or can be applied to select layers as needed toobtain the desired material properties for a particular application. Ina non-limiting embodiment, ultrasonic deep rolling at a frequency notexceeding 50 kHz promoted a desired ductility of at least 10%elongation.

Using the disclosed modified pulsed laser DED method, a titanium alloycan be directly deposited onto an aluminum substrate forming aprotective sheath having low galvanic interaction with the underlyingaluminum substrate and high strength. The disclosed process produces atitanium-based bulk metallic glass (BMG), which has an amorphousmicrostructure and which has been shown to result in a 3-5 order ofmagnitude reduction in electrical conductivity over current titaniumsheaths. The BMG can provide strength and protection to the componentwhile reducing or eliminating the need for an insulating layer.Crystalline phases introduced into layers of the BMG using ultrasonicvibration during deposition and deep rolling increase ductility of theprotective titanium element.

DISCUSSION OF POSSIBLE EMBODIMENTS

The following are non-exclusive descriptions of possible embodiments ofthe present invention.

A method of providing a protective titanium layer to an outer surface ofan aluminum component includes providing an aluminum component andforming a first layer of titanium-based bulk metallic glass on thecomponent, wherein formation of the bulk metallic glass layer comprisesdepositing a titanium alloy powder using pulsed directed energydeposition.

The method of the preceding paragraph can optionally include,additionally and/or alternatively, any one or more of the followingfeatures, configurations and/or steps:

A further embodiment of the foregoing method can include applyingvibrational energy to the component during deposition of a second layerof the titanium alloy powder to introduce crystalline phases into thesecond layer.

A further embodiment of any of the foregoing methods, whereinvibrational energy can be applied by an ultrasonic transducer.

A further embodiment of any of the foregoing methods, whereinvibrational energy can be applied intermittently to form bothcrystalline phases and amorphous phases in the second layer.

A further embodiment of any of the foregoing methods can further includedepositing a magnesium-containing powder using directed energydeposition to form an insulating layer between the component and thefirst layer of titanium-based bulk metallic glass.

A further embodiment of any of the foregoing methods, wherein formingthe first layer of titanium-based bulk metallic glass can includesimultaneously depositing a magnesium-containing powder using pulseddirected energy deposition.

A further embodiment of any of the foregoing methods, wherein the firstlayer of titanium-based bulk metallic glass can include a plurality ofsub-layers and wherein depositing the magnesium-containing powdercomprises depositing an amount of magnesium-containing powder thatdecreases with successive sub-layers.

A further embodiment of any of the foregoing methods can include forminga third layer of titanium-based bulk metallic glass on the component,wherein the second layer is formed between the first layer and the thirdlayer.

A further embodiment of any of the foregoing methods, wherein the secondlayer can have a tensile ductility of at least ten percent.

A further embodiment of any of the foregoing methods can further includemodeling a target microstructure for each of a plurality of titaniumalloy layers and devising a deposition process based on themicrostructure model, wherein devising the deposition process comprisesdetermining a schedule for applying vibrational energy.

A further embodiment of any of the foregoing methods, wherein thetitanium alloy powder has a melting point not exceeding 250 K greaterthan a melting point of the aluminum component.

A further embodiment of any of the foregoing methods, wherein thecomponent can be an airfoil and the titanium-based bulk metallic glasslayer is formed on a leading edge of the airfoil.

An aluminum component includes an outer surface and a sheath covering atleast a portion of the outer surface, wherein the sheath comprises atitanium-based bulk metallic glass layer characterized by apredominantly amorphous microstructure.

The method of the preceding paragraph can optionally include,additionally and/or alternatively, any one or more of the followingfeatures, configurations and/or components:

A further embodiment of any of the foregoing aluminum component, whereinthe sheath can further include a titanium-based bulk metallic glasslayer having both amorphous and crystalline phases.

A further embodiment of any of the foregoing aluminum components,wherein the sheath can further include a titanium alloy layercharacterized by a predominantly crystalline microstructure

A further embodiment of any of the foregoing aluminum components,wherein the titanium-based bulk metallic glass layer can be locatedbetween the outer surface and the titanium alloy layer characterized bya predominantly crystalline microstructure.

A further embodiment of any of the foregoing aluminum components caninclude a layer comprising magnesium formed between the outer surfaceand the titanium-based bulk metallic glass layer.

A further embodiment of any of the foregoing aluminum components,wherein the titanium-based bulk metallic glass layer can include aplurality of sub-layers comprising magnesium, wherein each successivesub-layer comprises a reduced amount of magnesium.

A further embodiment of any of the foregoing aluminum components,wherein the component can be an aluminum blade of a gas turbine engineand wherein the sheath covers a leading edge of the blade.

A further embodiment of any of the foregoing aluminum components,wherein the sheath has an electrical conductivity at least three ordersof magnitude less than a sheath formed of a non-bulk metallic glasstitanium or titanium alloy.

In another aspect, a method of providing a protective titanium layer toan aluminum component includes depositing a titanium alloy powder on anouter surface of the aluminum component, pulsing energy directly to thetitanium alloy powder, melting the titanium alloy powder, and forming afirst layer of titanium-based bulk metallic glass on the component.

While the invention has been described with reference to an exemplaryembodiment(s), it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment(s) disclosed, but that theinvention will include all embodiments falling within the scope of theappended claims.

The invention claimed is:
 1. An aluminum component comprising: an outersurface; and a sheath covering at least a portion of the outer surface,wherein the sheath comprises: a first titanium-based bulk metallic glasslayer characterized by an amorphous microstructure; and a secondtitanium-based bulk metallic glass layer having a microstructureincluding both amorphous and crystalline phases, wherein the crystallinephases are present in an amount providing the sheath with a tensileductility of at least ten percent; wherein the first titanium-based bulkmetallic glass layer is located between the outer surface and the secondtitanium-based bulk metallic glass layer.
 2. The aluminum component ofclaim 1, and further comprising a layer comprising magnesium formedbetween the outer surface and the first titanium-based bulk metallicglass layer.
 3. The aluminum component of claim 1, wherein the sheathhas an electrical conductivity at least three orders of magnitude lessthan 5.8×10⁵ Siemens/m.
 4. The aluminum component of claim 1, whereinthe first titanium-based bulk metallic glass layer is formed from atitanium alloy powder having a melting point not exceeding 250 K greaterthan a melting point of the aluminum component.
 5. The aluminumcomponent of claim 1, wherein the component is an airfoil and the sheathis disposed on a leading edge of the airfoil.
 6. The aluminum componentof claim 1, further comprising a plurality of second titanium-based bulkmetallic glass layers, wherein each second titanium-based bulk metallicglass layer has a thickness of at least 20 microns.
 7. An aluminumcomponent comprising: an outer surface; a sheath covering at least aportion of the outer surface, wherein the sheath comprises a firsttitanium-based bulk metallic glass layer characterized by an amorphousmicrostructure; and layer comprising magnesium formed between the outersurface and the first titanium-based bulk metallic glass layer.
 8. Thealuminum component of claim 7, wherein the sheath further comprises asecond titanium-based bulk metallic glass layer including both amorphousand crystalline phases, wherein the first titanium-based bulk metallicglass layer is located between the outer surface of the aluminumcomponent and the second titanium-based bulk metallic glass layer.